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Bev
220

Bev 220

Okay, so here are pics of the agatized Pentamerid brachiopod internal mold beside the rock we are discussing. I only reduced these to 40% so you should get a lot of detail and it will take several replies.

So this brach is supposed to be "agatized" as opposed to the rock which is agate (?). So yes a fossil can be inside a lava flow and create a void that can become filled with the substance that makes agates. You can have a fossil that is an agate...

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Auspex 3,602

Auspex 3,602

I think there is a distinction between "Lake Superior agates" and "agatized fossils from Lake Superior", in terms of origin. The former is created as such, within a void, and the latter is the result of replacement. I wonder whether the results might not be mineralogically distinctive too?

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Mr_ed
14

Mr_ed 14

I think there is a distinction between "Lake Superior agates" and "agatized fossils from Lake Superior", in terms of origin. The former is created as such, within a void, and the latter is the result of replacement. I wonder whether the results might not be mineralogically distinctive too?

That is an interesting thought.. I would think there would be some difference considering that there would me some kind of mineralization already present where the fossil is replaced .. but what?

Well let's put it this way.. I have to assume that you did a proper hardness test.. if it can't be scratched with a knife then it has to be agate or chalcedony or chert... I have to add that it still looks like onyx to me but I concede that there could be an agate that looks like onyx and that there could be chert that looks like agate(doubtful) so.. if you are ready to accept it as an agate and it is in your hand then it is an agate by default.. because of an inability to conclusively say otherwise from a picture. (Clear as the mud that makes beautiful jasper and chert huh?)

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Wrangellian
1,518

Wrangellian 1,518

Ed, sounds like are you talking about travertine, eg. 'honey onyx', not the harder chalcedony version..

like this slice I scanned (I think it's from Golden BC area):

I think Bev did the scratch test and hers is harder (mine, and yours I assume, are the softer travertine). I am still unsure of whether Bev's chunk is agate or chert but I would guess it is agate based on the apparent translucency. I guess it could be called onyx (SiO2 type) if it fits one's definition of that...

Now I am also confused about the differences between replacement by 'microcrystalline quartz' and agate which fills in voids. (There are no fossils generally in volcanic rock, Bev, but a calcite fossil can be dissolved out of the surrounding matrix by groundwater leaching, and replaced by chalcedony/agate.. is that the same as microcrystalline qtz? I don't know, but I have always understood the process of agate formation was the same or similar, whether the void was a cavity left by a dissolved fossil in sedimentary rock or a 'bubble' hole in lava or whathaveyou.)

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Mr_ed
14

Mr_ed 14

Look at post #125. Those chessmen were sold to me as onyx.. they can be scratched with a knife.. hardness of 4 or less.. they polish nicely and are banded and are white... not honey colored.. They may be travertine.. that they call onyx for some reason but I can not imagine why they would do that.. at the same time I am at a loss for good knowledge about travertine.. so am at a disadvantage there..

As I stated to Bev.. I find that rock too far removed from anything I have seen that is agate to think it is agate by looking at it..the bands are piled too high it seems and no trace of different colors... As I said before .. if it was a test with two answers available.. 1. chert and 2. Agate I would pick No 2. because it looks more like agate than chert to me.. but there are people here that think it is chert and I am hard pressed to disagree with anyone who says he knows..at this point.

That last picture Bev posted did the rock some justice.. and maybe some of the guys that have commented will have another look.

It may very well be as Bev says .. a fossil made of chert .. I don't think I can come to any conclusion that I would be happy with without actually holding the rock and maybe not even then...I am satisfied that it is an agate that is very different from the norm.

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Tethys
43

Tethys 43

I went looking for other Mn varieties of chalcedony besides LSA, and found a good candidate for the source of Bevs agatized stromatoporoid.

The fossil bearing Paleozoic rocks of Mn are found mostly within a narrow structure known as the Hollandale embayment. It is thinnest in the north and gets progressively thicker as you go south. Lithology is highly variable Bev has late Ordovician and Devonian formations that are found only in extreme southern MN and north Iowa. K-bentonite layers are common and are interpreted as ash beds from the Appalachians, which were a volcanic island arc that was converging on Laurentia in the late Ordovician.

In the biostratigraphy literature found here on page 24 (pdf), the Maquoketa Formation is notable because the fossils are replaced with chalcedony.

Most of the Maquoketa in MN is classified as Elgin member. The Elgin is composed of thin bedded, light grey to yellowish grey, very fine grained to sublithographic fossiliferous limestone, interbedded with grey to brownish grey, unfossiliferous, shaly dolostone.

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Tethys
43

Tethys 43

I hate to introduce yet another term for SiO2, but using chalcedony to describe a distinct variety of chert seems to be the accepted scientific terminology.

There is a brief mention of the Ischadites iowensis zone in the paper, which nowadays refers to a green algae. In the original geological surveys that were done around the 1900s, both stromatoporoids and receptuculites were lumped in under Ischadites.

There is a old report written by Elkaniah Billings on late Ordovician stromes from lake Winnipeg. He made a special note that an aulacapora (sp?) specimen was so completely replaced with silica that it was not affected by acid and concluded that it was chalcedony. I apparently did not bookmark the page, unfortunately.

I also found a passing reference to local stromatoporoids in another paper on correlating the various Ordovician strata through "fingerprinting" the k-bentonite layers. It merely said that eight different varieties were known and nobody has studied them.

Edited June 11, 2014 by Tethys

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Bev
220

Bev 220

Tethys, "There is a old report written by Elkaniah Billings on late Ordovician stromes from lake Winnipeg. He made a special note that an aulacapora (sp?) specimen was so completely replaced with silica that it was not affected by acid and concluded that it was chalcedony."

So, a 100% conclusive test of this rock and others like it, would be to pour a bit of Muratic Acid on it. No effect and it would be either calcedony or quartz and certainly not onyx or travertine or chert?

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Wrangellian
1,518

Wrangellian 1,518

The terminology as I understand it, in a nutshell, is chalcedony = agate = quartz and 'true' onyx is a type of chalcedony/agate.

Travertine (the yellowish variety from Golden BC being 'honey onyx' to the rockhounds around here, but can be white or various other colors) = banded calcite.

The former of course is much harder and is not affected by muriatic acid. Agates/chacedony comes in many different colors and patterns but here we're concerned with the banded types (which also come in many colors, shapes, sizes) and how to distinguish it from chert which is another silicious rock that was originally sediments. Distinguishing it from travertine is easy and I think we've done that.